1. Field of the Invention
The present invention relates to an antilock brake system with separate front- and rear-wheel safety means, which system is equipped with independently operable front- and rear-wheel antilock control means so that it can always retain the brake force to the front wheel and maintain the steering stability of the wheeled vehicle even if either the front-wheel antilock control means or the rear-wheel antilock control means is troubled in its brake force suppressing state.
2. Description of the Prior Art
An antilock brake system is equipped with both brake force transmitting means for transmitting and applying the brake force to wheels and antilock control means for exerting such an action upon the brake force transmitting means as to suppress the brake force on the basis of predetermined controlling input factors when the brake force applied from that brake force transmitting means to the wheels is excessively strong. The antilock control means usually uses a fluid such as working oil as a control pressure transmitting medium, which is to be introduced through a pressure introduction valve into a control pressure chamber and released through a pressure release valve out of the control pressure chamber. The pressure introduction valve and the pressure release valve are respectively actuated by electromagnetic actuators, of which the electromagnetic actuator for the pressure introduction valve is made operative to open the pressure introduction valve, when energized, and to close it when deenergized whereas the electromagnetic actuator for the pressure release valve is made operative to close the pressure release valve, when actuated, and to open it when deenergized. As a result, when both the electromagnetic actuators for the pressure introduction and release valves are energized, the amount of the fluid in the control pressure chamber is continuously increased so that the brake force is reduced independently of the intention of a driver. On the other hand, when not the electromagnetic actuator for the pressure introduction valve but only the electromagnetic actuator for the pressure release valve is energized, the fluid in the control pressure chamber comes into its confined state so that the brake force is held at a constant level independently of the intention of the driver. Moreover, when both the electromagnetic actuators for the pressure introduction and release valves are deenergized, the fluid in the control pressure chamber is in its freely releasable state so that the brake force can be built up in accordance with the intention of the driver.
The respective electromagnetic actuators have their energizations controlled in accordance with the control output signals which are generated from antilock control circuits by conducting the arithmetic processings on the basis of predetermined control input factors such as the running speed of the vehicle, the accelerated or decelerated velocities of the wheels or the slip rates of the wheels.
It is generally customary to provide the antilock brake system with safety means for making the function of the antilock control means wholly ineffective, when the antilock control means is troubled, thereby to allow the antilock brake means to act as the normal brake means having no antilock control function.
Here, if all the antilock control functions to control the respective wheels of the vehicle are wholly or simultaneously made ineffective in accordance with the troubles of the antilock control means, the difference of the functions of the antilock brake system between the normal operation and the malfunction is enlarged to invite a fear that the driver feels anxious of that abrupt change in the performances. In the prior art, however, there is no suitable antilock brake system which has succeeded in solving that subject matter which can enjoy reliable operations and high utility although it has a simple construction.